1. Field of the Invention
This invention relates to a field-emission cold-cathode electron gun for forming an electron beam using a cold cathode of a field emitter array (FEA) type having a fine structure fabricated by thin film technology, and to a microwave tube including the cold-cathode electron gun, such as a traveling-wave tube (TWT) or a klystron.
2. History of the Prior Art
A field-emission cold cathode is disclosed in an article by C. A. Spindt, Journal of Applied Physics, Vol. 39, No. 7 (June 1968), pages 3504-3505, entitled "A Thin-Film Field-Emission Cathode." The field-emission cold cathode comprises a plurality of minute cold cathodes in an array fashion. Each minute cold cathode comprises a minute conical emitter (an electron emission electrode) and a gate electrode (a control electrode) formed in the vicinity of the emitter for extracting electrons from the emitter and for controlling the flow of the electrons. Such a field-emission cold cathode is called a Spindt-type cold cathode. It is desirable to use the field-emission cold cathode in a microwave tube such as a traveling-wave tube (TWT) or a klystron. This is because the microwave tube may be miniaturized due to the cathode not requiring a heater. However, in a case where the Spindt-type cold cathode is installed in an electron beam apparatus such as a microwave tube, there may be two problems: 1) electrons having a lateral velocity component, and 2) failure to meet the design conditions of a Pierce electron gun.
In order to resolve the first problem, a known method is to deposit a focusing electrode on the gate electrode to suppress the lateral velocity component. However, this method cannot suppress a ripple of the electron beam.
In order to resolve the second problem, another conventional field-emission cold cathode is disclosed in Japanese Unexamined Patent Publication of Tokkai No. Hei 7-14,501 or JP-A 7-14,501. The field-emission cold cathode according to JP-A 7-14,501 further comprises a peripheral electrode. However, this cathode cannot sufficiently suppress the lateral velocity component or completely suppress the ripple of the electron beam.
Another conventional hot-cathode electron gun is disclosed in an article by A. Starrans, et. al., PROCEEDING OF THE IEEE, VOL. 61, NO. 3 (MARCH 1973), pages 299-301, entitled "High-Power Linear-Beam Tubes." This hot-cathode electron gun comprises a ring-shaped hot cathode, a central beam control electrode, a cylindrical beam control electrode, and an anode. However, this hot-cathode electron gun cannot control the amount of current of the electron beam.
Another conventional field-emission cold cathode is disclosed in Japanese Unexamined Patent Publication of Tokkai No. Hei 5-307,930, namely, JP-A 5-307,930. This field-emission cold cathode includes a metal in a central part without forming any minute cold cathode. However, this field-emission cold cathode cannot suppress the lateral velocity component.
Another conventional field-emission cold cathode is disclosed in Japanese Unexamined Patent Publication of Tokkai No. Hei 6-243,777 or JP-A 6-234,777. This field-emission cold cathode includes a shielding electrode kept apart from a plurality of ring-shaped gate electrodes by ring-shaped grooves. The shielding electrode is supplied with a shielding voltage while each ring-shaped gate electrode is supplied with a gate electrode voltage not higher than the shielding voltage. However, this field-emission cold cathode cannot suppress the lateral velocity component.